The figure shows that the applicator generally takes the form of a pistol and comprises a casing 1 enclosing a tubular heating body 2 through which passes an elongate chamber 2 having a first end 4 shaped as an inlet for a stick B of hot melt glue and a second end 5 equipped with a molten glue dispenser nozzle 6. The applicator further includes means (not shown) for pushing the stick B progressively into a bush 7 coaxial with the chamber 3 and then into the chamber, and electrical means for heating the portion of the glue stick that is pushed into the chamber.
The applicator further comprises a handle 8 equipped with a trigger 9 for operating the means for pushing the stick, which means can comprise a mechanical transmission using a linkage or a rack, for example. This is well known in the art.
The electrical means for heating the mass of glue contained in the chamber 3 are supplied with power via electrical wires 10 connected to a terminal block 111 of a circuit 11 for regulating the power supply to the heating means. The circuit 11 is itself supplied with electrical power via a cable 12 connected to a terminal block 112 of the circuit 11 and to the AC mains. Alternatively, the circuit 11 could also be supplied with power by a storage battery internal or external to the applicator.
According to the U.S. patent previously cited, the heating means comprise one or more heating elements each comprising a track of a material with a high electrical resistance and provided with means for connecting it to an electrical power supply, the track being formed on an electrically insulative surface of a substrate arranged to be in thermal contact with the mass of glue contained in the chamber 3.
This kind of flat heating element can be produced using means usually employed for the fabrication of thick film hybrid circuits, which comprise a substrate of alumina, enameled sheet metal, or stainless steel sheet, for example, covered with a dielectric, a track in the form of a paste being screenprinted onto the substrate and, after drying, constituting a resistive “ink” adapted to heat an adjacent mass of glue by the Joule effect and thermal conduction, when an electrical current flows through it.
These heating elements have many advantages over the electrical heating means conventionally used in hot melt glue applicators. They heat up very quickly, because of their low thermal inertia. Their heating power per unit surface area is very high. Furthermore, screenprinting allows great variation in the design of the tracks. This facilitates the adaptation of such heating elements to specific or new requirements.
The patent previously cited describes various embodiments of a hot melt glue applicator equipped with the above kind of heating elements, in which the elements line all or part of the wall of the chamber 3, to establish optimum thermal contact with the mass of glue contained in the chamber. The heating body 2 is made from a thermally insulative material in this case.
This arrangement of the heating elements nevertheless has disadvantages. Apart from the fact that they are inconvenient to install, supplying electrical power to them requires holes in the heating body for the power supply wires to pass through, which holes can give rise to leakage of the glue, especially if the wires also pass through the substrate of the heating element. If the latter is made of metal, the passage of the wires further gives rise to problems of electrically insulating the wires from the substrate. Finally, if the screenprinted tracks face the mass of glue contained in the chamber 3, care must be taken to ensure that the spot welds connecting the electrical power supply wires to the tracks do not impede the movement of the mass of glue in the chamber.